The present invention relates generally to a process for making gypsum panel, and more particularly, to a process for making thin, sealant-coated, fiber-reinforced gypsum panel and the thin, sealant-coated, fire and indentation resistant gypsum panel made thereby.
Conventional gypsum wallboard or panel is typically manufactured from a plaster slurry wherein a wet slurry of gypsum, generally referred to as calcined gypsum, is placed between two layers of paper and the slurry is allowed a certain amount of time to set. The set gypsum is a hard and rigid product obtained when the calcined gypsum reacts with water to form calcium sulfate dihydrate. Calcined gypsum is either calcium sulfate hemihydrate (CaSO.sub.4. 1/2H.sub.2 O) or calcium sulfate anhydrite (CaSO.sub.4). When calcium sulfate dihydrate is heated sufficiently, in a process called calcining, the water of hydration is driven off and there can be formed either calcium sulfate hemihydrate or calcium sulfate anhydrite, depending on the temperature and duration of exposure. When water is added to the calcined gypsum to cause the gypsum to set, in essence, the calcined gypsum reacts with water, and the calcined gypsum is rehydrated.
In typical gypsum wallboard, the two layers of paper contain the slurry and provide the strength required in installation and use. The wallboard is cut into discrete lengths to accommodate subsequent handling and then dried in heated dryers until the board is completely dry.
The bending strength of the wallboard depends on the tensile strength of the paper. The gypsum serves as a "spacer" and accounts for fire resistance and moisture absorbing and moisture releasing activities. The static properties are limited, and the surface treatment and the joint filler system are determined by the paper.
Although paper-covered wallboard has many uses and has been a popular building material for many years, it would be advantageous to provide gypsum panel which did not rely on paper for strength and other properties. A "faceless" water-resistant gypsum product is referred to in U.S. Pat. No. 5,135,805 and is defined as that does not include a facing sheet of paper, glass mat or similar material. According to U.S. Pat. No. 5,135,805, which is incorporated by reference herein in its entirety, such products typically contain reinforcing fibers, for example, cellulosic fibers, such as wood or paper fibers, glass fibers or other mineral fibers and polypropylene or other synthetic resinous fibers, and the reinforcing fibers can be about 10 to about 20 wt. % of the dry composition from which the set gypsum product is made. The density of such a product is typically within the range of about 50 to about 80 pounds/cubic ft. As known in the art, such "faceless" gypsum products can be formed from dry or aqueous-based compositions.
U.S. Pat. No. 5,135,805 refers to a process of incorporating siloxanes as water-resistant additives into water-resistant gypsum-based articles, particularly, an article which can be used to excellent advantage as the core of water-resistant gypsum wallboard, by adding the siloxane to water (including other optional liquid ingredients such as calcium lignosulfonate); mixing said siloxane/water admixture with calcined gypsum to form an aqueous slurry; and shaping and allowing said slurry to set to form a set gypsum based, water-resistant article.
Compositions containing calcium sulfate in hemihydrate or anhydrous form and fibrous additives, including paper fibers preferably derived from waste newspaper, capable of being formed into an unfaced building material having certain mechanical properties and/or fire resistance are referred to in U.S. Pat. No. 5,171,366 which is incorporated by reference herein in its entirety. In U.S. Pat. No. 5,171,366, paper fiber is combined with the calcium sulfate in the form of a pulp of paper stock that contains at least about 20 times more water than paper stock, wherein an aqueous dispersion, i.e., a slurry, of solids includes by weight about 53% to about 78% calcium sulfate, about 7% to about 30% cellulosic fiber, and preferably about 1.5% to about 35% performance booster selected from inorganic fiber, clay, vermiculite and binder polymer, together with a quantity of water at least about 25-fold in excess of that required to completely hydrate the calcined gypsum.
U.S. application Ser. No. 07/572,758, filed August 23, 1990, now U.S. Pat. No. 5,342,566, and assigned to the assignee of the present invention and a co-assignee by Schafer et al., and which is incorporated by reference herein in its entirety, refers to a method of producing fiber gypsum board comprising the steps of mixing in a preliminary mixing step predetermined amounts of fibers and water respectively, to form a mixture of wetted, loose fibers; mixing in a mixing step the wetted fibers with a predetermined amount of dry calcined gypsum; premixing an accelerator with one of the components of dry calcined gypsum, fiber and water; promptly laying the mixed composition into a matt; immediately degassing the matt in a first decompression step, adding a predetermined amount of water onto the resultant matt; and immediately compressing the matt to form a board composed of bonded fibers and gypsum. Schafer et al. refer to a homogeneous board which is preferably a gypsum board reinforced by fiber, such as paper fiber, wherein several layers of board forming materials are placed on each other before the board is fully formed, pressed, and dried, and wherein each of the layers is identical in composition.
Although new and improved gypsum panels can be made by the process of Schafer et al., it is always desirable and advantageous to improve the strength and other properties of gypsum panels. For example, it is advantageous to provide gypsum panels less than 0.50 inch, and preferably, less than 0.375 inch, in thickness, i.e., thin gypsum panels, having high strength and which are fire and indentation resistant and resistant to moisture absorption. Such panels would be excellent substitutes for traditional products, such as wood, currently used, for example, for floor underlayment, soffit boards and the like.